Despite the advances of modern medicine, getting sick is still something most people fear. Most frightening is what we don't know, the mysteries of the human body, and all of its surprises. Some of the biggest medical mysteries can be found among the 6,000 diseases classified as rare by the National Institutes of Health.

Today, we examine the rare, the mysterious, and the unknown, beginning with one condition at the center of a heated debate because doctors can't even agree that it exists. Here's Elizabeth Cohen.

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UNIDENTIFIED FEMALE: Kind of bluish.

UNIDENTIFIED MALE: It's kind of a semicircle. And the top is kind of reddish.

UNIDENTIFIED FEMALE: The very first one was kind of a creamy white. COHEN: People claim they have strings, not hairs, but strings growing out of their skin. Doctors and nursing can't believe what they're seeing.

UNIDENTIFIED MALE: They are blue, they're red, they sometimes move, which is really bizarre.

UNIDENTIFIED FEMALE: This didn't seem like anything I had ever seen that was coming out of a human body.

UNIDENTIFIED MALE: This is a mystery. It really is.

COHEN: The mystery starts here in west Texas.

CAROL ARLEDGE, MORGELLON'S PATIENT: Good boy. Put your nose in there.

COHEN: Rancher Carol Arledge was one of the first people to see white fibers and black specks popping out of her skin. She went to her dermatologist.

ARLEDGE: And she said I can't believe you did this to yourself. It's like I didn't. And I said well do you want me to come back if it doesn't get better? She said no, if it doesn't get better you need to find a psychiatrist.

COHEN: So did Carol Arledge do this to herself?

She says no.

UNIDENTIFIED FEMALE: You've still got some active lesions there.

COHEN: And did Marnie Winkie scratch these sores into her own face? She also says no. Carol, Marnie, and thousands of others from various parts of the country complain of similar ailments. They say it feels like something's crawling beneath their skin, and that they're sick and exhausted.

The patients call it Morgellon's disease. On Web sites, people post pictures they say prove it's a real illness. Now even the Centers for Disease Control is looking into it.

GINGER SAVELY, NURSE PRACTITIONER: These lesions on your head, just so I can clarify this...

COHEN: Nurse practitioner Ginger Savely was one of the first people to treat Morgellon's patients.

SAVELY: That's not from you gouging or scratching.

COHEN: You know that all of this sounds kind of crazy.

SAVELY: Yes, but I know what I see with my own eyes.

COHEN: The mystery of Morgellon soon caught the attention of a Tulsa, Oklahoma researcher, Dr. Randy Weinmore. His search began here.

DR. RANDY WEINMORE: Here's a lovely red.

COHEN: He wondered could the fibers in the skin simply have rubbed off of people's clothing?

WEINMORE: Here's a nice blue material that probably will have some loose fibers that can be pulled off with scotch tape.

COHEN: When you took all of these clothing fibers and put them underneath a microscope, did it look like the fibers underneath the skin of the Morgellon's patients?

UNIDENTIFIED FEMALE: That is definitely not a hair. It's a blue thing there.

COHEN: We were there one day when they brought in four patients.

UNIDENTIFIED FEMALE: I want to see if I can get the little purple looking things.

COHEN: Extracted fibers from under their skin.

UNIDENTIFIED FEMALE: I got it.

COHEN: Made slides and looked at them under a microscope.

UNIDENTIFIED FEMALE: Oh, it's red.

UNIDENTIFIED MALE: Actually it's red and blue.

COHEN: And this is what they saw -- black, red and blue fibers lurking under the skin. Now they've seen about 25 patients. And the OSU doctors are convinced Morgellon's is real, but the medical establishment says they are wrong.

SCHEINFELD: It's all in their heads. This is somebody who's picking at themselves. And people can pick at themselves for all kinds of reasons.

COHEN: But how does he explain the fibers?

Dr. Scheinfeld says once the patients create a sore, they shove fibers into it.

UNIDENTIFIED FEMALE: There was absolutely nothing on the surface there.

COHEN: But the OSU doctors say that's impossible. They say they found most of the fibers away from the sores under unbroken, smooth skin.

UNIDENTIFIED MALE: We take this piece of skin, and then we extract genes from them.

COHEN: This biologist at SUNY-Stonybrook says he found a gene that only exists in plants inside the skin of the Morgellon patients. Many of the people who claim to have Morgellon's have spent time working in the soil.

Do you think in a couple of years you might be eating your words? Maybe research will show you're wrong?

SCHEINFELD: In this case, I don't think so.

DR. RHONDA CASEY, OKLAHOMA STATE UNIVERSITY: I would challenge any of these physicians who think that we are just feeding into the delusions to come and examine a group of these patients and see what I've seen.

COHEN: Delusion or disease? The threads of this argument continue on.

Elizabeth Cohen, CNN, Tulsa, Oklahoma.

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GUPTA: Well, Elizabeth, that is fascinating. And the CDC is now in talks about conducting field tests to try and nail down whether Morgellon's is truly a new disease.

Coming up on HOUSE CALL, a disease that leaves you desperately trying to feel anything.

UNIDENTIFIED FEMALE: Playing in a pain-free world. Idyllic? More like deadly. Inside Roberto's world when HOUSE CALL returns.

First, answer this. Which of these is a rare disorder? A, Blue Diaper syndrome? B, Job syndrome? C, Jumping Frenchmen of Maine. D, all the above. That answer after the break.

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UNIDENTIFIED FEMALE: Before the break we asked, which of these is a rare disorder? A, Blue Diaper syndrome? B, Job syndrome? C, Jumping Frenchmen of Maine. D, all the above.

The answer D, all of the above.

GUPTA: And for more information on these conditions, go to the National Organization for Rare Disorders at rarediseases.org.

Now we move on to a cluster of disorders that cut off a person's pain reflex. That doesn't sound bad, right? No pain? But everything in our body has a purpose. So whether it's a broken foot, even choking, the boy you're about to meet won't feel a thing, won't feel his body screaming to stop.

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UNIDENTIFIED FEMALE: Put that down.

GUPTA (voice-over): For most four-year-olds, a bump on the head is followed by an ouch and a painful cry. Not so for Roberto Salazar, who is almost always smiling. Even as a newborn, Roberto never did cry, even when he was being given a shot or when he should have been hungry.

SUSAN STINGLEY-SALAZAR, ROBERTO'S MOTHER: Eating is supposed to be this inborn thing that everyone wants to do, that you know, that everyone knows how to do.

GUPTA: At eight months, he weighed only 12 pounds. A stomach tube was inserted to force calories into him and to keep him alive. His family knew something was terribly wrong, but were shocked when he began teething.

SALAZAR: Roberto didn't like any teething kind of toys. It was always his hands. Hands in the mouth constantly. And then, he would get his hands bleeding. He had big open sores on his hands.

GUPTA: In desperation, Roberto's mother took him to see dozens of doctors. None had helpful answers. Susan found Dr. Felicia Axelrod, a specialist in a very rare set of disorders.

DR. FELICIA AXELROD, NYU DYSAUTONOMIA CENTER: All of the children with HSAN live in a world without pain. The body wants to have input. The body needs sensation to come into it so that it can process, so it can develop, so the brain can be stimulated.

SALAZAR: HSAN or Hereditary and Sensory Autonomic Neuropathies is a group of seven rare disorders. It affects the autonomic nervous system, a system regulating blood pressure, heart rate, sweating, and tearing in the eyes. At varying degrees, children with these diseases feel very little to no pain at all.

AXELROD: When one hears that the child lives in a world without pain, you would think that this is an idyllic world. But actually, pain is a very important function. It helps us protect ourselves from injury, to know when we should stop doing something.

SALAZAR: More? You want more tickles.

GUPTA: Roberto can feel touch, even tickles on the tummy. But he does not feel pain. He cannot sweat or regulate his own body temperature, making air conditioning in the summer a matter of life or death. Roberto's type is the most severe and rare type of HSAN. More than half its victims die by age 3. Ironically, the severe condition often is also combined with hyperactivity. Roberto now takes Ritalin.

SALAZAR: If you can imagine lacking the ability to feel pain and hyperactivity. It's the deadliest combination that you could put together, especially for a child.

GUPTA: Doctors say Roberto will be wheelchair bound by the age of ten, a result of the wear and tear he does to his own body every day.

There are only 17 surviving children in the United States with this condition. There is no cure or treatment. Therapy deals with the symptoms, not stopping the disorder.

But there is some hope in the form of older HSAN kids, like 15- year-old Perry Goldberger. She has lived her entire life with familial dysautonomy, a less severe form of HSAN.

PERRY GOLDBERGER, HSAN PATIENT: You get hurt sometimes. My parent would say, where did you get that? And I was like, I don't know. But it's also like a bonus, because then you don't have to feel pain. And so it doesn't hurt.

This is my schedule, my daily schedule that I take every single time. You get used to it after a while.

GUPTA: For young HSAN patients, these are hard but vital lessons to learn.

SALAZAR: He isn't ever going to learn to feel pain, but he can learn what not to do, not to get into that situation of pain. And it's hard.

GUPTA: For kids like Perry and Roberto, it's going to be a life- long struggle, but it is possible to live a happy, albeit pain-free, life.

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GUPTA: Well, this group of genetic disorders are recessive, meaning both parents have to carry the genetic mutation in order to pass it on to a child. There's still less than a one in four chance that the child will develop it.

When HOUSE CALL returns, a prison of your own body's making.

UNIDENTIFIED FEMALE: Joints fusing together, muscles locking into place, eventually complete immobility. Inside the rarest of rare disorders coming up on HOUSE CALL.

First, this week's medical headlines in "The Pulse."

(BEGIN VIDEOTAPE) JUDY FORTIN, CNN CORRESPONDENT (voice-over): A simple gel or cream may help prevent HIV infection in women. Microbicides won't be available for several years, but researchers say early studies show promising results. Millions of AIDS deaths worldwide could be prevented if more boys were circumcised. This coming from experts attending the international AIDS Conference in Toronto.

A new study reveals men with less aggressive prostate cancer may be overtreated. University of Michigan researchers found of 25,000 low risk patients, 55 percent were treated with surgery or radiation. Given the risk of complications from such treatments, ask your doctor if a wait and see approach called active surveillance may be a reasonable option.

Judy Fortin, CNN.

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GUPTA: Now if you missed anything from today's show, don't worry. You can now get a free transcript. Click on our Web page at CNN.com/HouseCall, and you'll find a library of transcripts, along with a link that enables you to send us questions.

It's one of the rarest conditions known to man. As it progresses, bone grows where it was never intended to. It locks up the joints. Eventually, it can leave a person rigid.

In a race against time, the cause has been found. But for the few waiting, the cure could still be years away. Just a warning now, some images in this report could be disturbing.

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GUPTA (voice-over): Stephanie Snow is visiting the University of Pennsylvania School of Veterinary Medicine. It's a place she eventually hopes to make her home.

STEPHANIE SNOW, LIVING WITH FOP: My career goal is to be a veterinarian at the University of Penn some day.

GUPTA: The visit encourages her to dream of the future, but the campus holds another hope for Stephanie, a very personal one. It's where a team of scientists are racing to find the key to unlocking Stephanie's body and literally setting her free.

SNOW: FOP stands for Fibrodysplasia Ossificans Progressiva, but we like to call it FOP for short. It's a lot easier.

GUPTA: She jokes about it today, but FOP is an age old medical mystery. Muscles, tendons, and ligaments throughout her body slowly turn to bone. When Stephanie was born, only her big toes bore the telltale signs of all children born with FOP.

JENNIFER SNOW, STEPHANIE'S MOTHER: They were both shortened, and just -- they were cute, but funny. And always kind of questioned it and wondered why they were that way. And nobody really had an answer for us.

GUPTA: But soon, the signs began to multiply.

J. SNOW: And it wasn't until she was a year and a half old when she had a very unusual lump grow on the back of her neck that we began this whole journey with FOP.

GUPTA: The disease is one of the rarest conditions known to mankind. Only one in two million people have it. About 90 percent of the time, the lumps are first mistaken for cancerous tumors. But in fact, each lump represents the painful episodic progression of the disease.

Dr. Fred Kaplan, an orthopedic surgeon and founder of the FOP Research Program at the University of Pennsylvania, describes the formation of bone as explosive in nature.

DR. FRED KAPLAN, UNIV. OF PA. SCHOOL OF MEDICINE: It happens within hours. Very often a child will go to sleep feeling fine. And the next morning, a mother will -- the child will wake up and the mother will notice that there's painful swellings that look like tumors that are on the neck, or back, or limbs of this child that weren't there the night before.

GUPTA: Any attempt to surgically remove the extra bone only ignites even more painful bone growth. Despite the fact that her shoulders, neck, and part of her knees have already fused, at age 15, Stephanie leads a full life. She raises rabbits and lambs with the 4- H Club. And until recently, was a cheerleader for her community lead.

S. SNOW: Is regular cheerleading but with no stunting. It's like I have mobility, but it didn't really affect it that much. And I had a lot of support of the team.

GUPTA: But you see, every day is a step toward the eventuality of FOP. By their mid 20s most people with the condition have formed a full second skeleton becoming forever immobile.

S. SNOW: I just don't think about it till it comes. So I just kind of just put that aside and just do whatever I'm doing.

GUPTA: After 15 years of research, Dr. Kaplan's team has finally made a discovery. They have pinpointed the cause of the disease. It's a single genetic mutation in just one of six billion nucleotides that make up the human genome.

The next step -- creating a drug that will switch off the genetic signal that's causing the bone formation.

KAPLAN: My dream is to be able to go in and to be able to remove this extra bone under the cover of these medications, and liberate those frozen joints, and set these children free.

GUPTA: And that hope could be less than a decade away. For Stephanie, it means an even greater chance of fulfilling her dream as a veterinarian.

S. SNOW: Hopefully, they'll have a cure by then, but it just all depends. And I mean, it will work out eventually somehow, but there will be some bumps in the road. But they're always able to fix it.

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GUPTA: Stephanie has such a great attitude. We wish her all the best and the same to the researchers now searching for that cure.

Coming up on HOUSE CALL, the American dream backfires for some new immigrants. Find out how.

UNIDENTIFIED FEMALE: From deprivation to abundance, how living in the land of plenty sets some up for a lifelong struggle.

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GUPTA: Welcome back to HOUSE CALL. We've been talking about medical mysteries. But here's something we do know. Obesity contributes to some of the deadliest diseases in this country. And unfortunately, overweight and obesity are increasingly becoming the norm in America, even for new Americans.

Studies show immigrants come here a healthy weight or even malnourished, but they soon become overweight after being here only a few years.

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UNIDENTIFIED FEMALE: OK, so let's get started.

GUPTA (voice-over): As these immigrants who have recently arrived in the U.S. settle into the new homeland, they risk picking up some bad American habits like eating too much.

BINDI LESSING, DIETICIAN: My name is Bindi. Welcome to America. And we're going to talk a little bit about nutrition.

GUPTA: So dietician Bindi Lessing lectures her class at the Hartland International Refugee Center in Chicago on how to fight fat in the land of plenty. Plenty of junk food, that is.

LESSING: What about candies? Where does that go? Cookies? Where does the candies? Where do we put that? How much of this should we eat?

GUPTA: It isn't easy to avoid the lure of bad American cuisine. Just ask Francois Adentur, who immigrated from Togo.

LESSING: He said he's discovered foods like pizza that taste really good, but can definitely make you fat.

GUPTA: Adentur has been listening to Lessing, but many immigrants in the program tell her they've never tasted junk food before they got here. LESSING: I had no clue sugar, you know, was energy. And they -- you know, they tell me I didn't that it was bad for me.

This is delicious, but you eat too much, it's a problem. Who knows how to do the hula-hoop like this?

GUPTA: Lessing also teaches these families simple exercises they can do without a backyard.

She says they struggle to balance their new diets with their new, more sedentary American lifestyles.

LESSING: They used to work a lot. They used to walk a lot and be active all day. And now it's like they're, you know, either sitting at school, or sitting, you know, when they come home. They're cooped up in the house.

GUPTA: Lessing found space in a community garden for these families to grow their own herbs, fruits, and vegetables. While they garden, they burn calories as well.

All in the hope that as these immigrant families live the American dream, they don't acquire the American bulge.

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GUPTA: Well, that's all the time we have for today. Next weekend, make sure to tune in. It's time for your kids to be heading back to school. That can sometimes mean tough times for parents and for their children. From anxiety to sleep problems, to dealing with the lack of school nurses.

Tune in for our special back to school edition of HOUSE CALL. That's next weekend at 8:30 Eastern. Thanks for watching. I'm Dr. Sanjay Gupta. Stay tuned now for more news on CNN.

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